Place castings in the bucket by either:
putting the castings into a thin mesh "teabag" of pantyhose or a similarly sheer sock and knotting the end. The knotted end of the bag can be hung down and submerged so the tea bag is situated above the rising bubbles. Some simply dump the teabag in.
putting the castings directly in the bucket (with no tea bag) if you plan to use a watering can anyway, otherwise, straining through cheesecloth or mesh may be necessary when using backpack-type sprayer with nozzles that could clog easily with debris and detritus.